A need for electronic systems with one hundred percent on-line performance, translates into a need for live insertion and removal capabilities in electronic assemblies and/or sub-assemblies. Electronic assemblies, such as in communication network controllers, servers, gateways, routers and the like, which need to be on-line continuously require the ability to have sub-assemblies inserted and removed from the system without having to power the system down, install or remove the sub-assembly, re-apply power and re-initialize the system.
Several criticalities need to be considered in implementing a scheme to enable insertion and removal of sub-assemblies from a live (powered-up) electronic assembly. The sub-assembly which is unpowered and being inserted into the live assembly will, initially, draw considerable amounts of current from the power system supplying the live assembly. Similarly, a sub-assembly being removed from the live assembly may considerably decrease a load seen by the power system. Thus, there is a need to provide a means for assuring that the sub-assembly being inserted or removed from the powered assembly does not significantly disturb a system's power source. Further, when a sub-assembly is withdrawn under powered-up conditions current continues flowing across gaps created between assembly and sub-assembly contact points, which creates arcing. Such arcing causes deterioration of contact surface coatings and can damage the contact surfaces.
In systems that have parallel or serial data paths and associated control signals, consideration must also be given with respect to signal paths and buses. An unpowered, uninitialized sub-assembly contacting a powered bus can corrupt signals on the powered-up paths. When bus cycles are lost because data or control signal integrity is not maintained, it is extremely difficult to recreate the lost bus cycles. Thus, it is desirable to preclude the loss of bus cycles caused by insertion or removal of a sub-assembly.
An additional consideration that arises when insertion or removal of a sub-assembly to or from a powered assembly is desirable, is protection of the circuitry on the sub-assembly being inserted or removed. Many integrated circuits are fabricated such that there is a low impedance path between a circuit output and Vcc in a back biased direction. When such a circuit is present on a sub-assembly that is inserted into a live assembly, current from the live assembly, seeking the path of least resistance, will surge through the low impedance path possibly destroying the device. Some integrated circuit manufacturers put a diode in the low impedance path in certain logic families. However, often these diodes are not tested and one cannot be certain of their presence or functionality.
The aforementioned considerations are presently addressed in part by some implementations of live-insertion schemes known in the art. One implementation known, is to use DC-DC conversion schemes wherein a backplane is provided with a backplane voltage at a higher voltage than is used by the sub-assemblies. Each sub-assembly has a DC-DC converter for connection to the backplane voltage, to effect power isolation. Such an implementation requires significant additional and costly circuitry and consumes considerable space on the sub-assemblies.
An alternative scheme using a mechanical switch to remove and supply power to a sub-assembly being removed or inserted, is disclosed in U.S. Pat. No. 4,835,737. The modules to be live inserted or removed each have an associated mechanical switch that causes an inhibit signal to be issued to a control circuit which has a resident state machine to effect a logic sequence that inhibits bus operation, such as by halting clock signals. Two different lengths of pins are used in a connector to which the actuator operated switch is connected to assure that the switch state indicating pins contact the connector before other signal pins. However, in such an embodiment no attempt is made to protect voltages on the backplane or to protect devices on the sub-assembly being inserted or removed. Further, such a live insertion mechanism requires elaborate sequential logic circuitry.